The present invention relates to an apparatus and process for dispensing material, and more specifically to an apparatus and process for dispensing solder paste in a screen or stencil printer.
In typical surface-mount circuit board manufacturing operations, a stencil printer is used to print solder paste onto a circuit board. Typically, a circuit board having a pattern of pads or some other, usually conductive, surface onto which solder paste will be deposited is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to properly align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. In some prior art systems, an optical alignment system is used to align the circuit board with the stencil. Examples of optical alignment systems for stencil printers are described in U.S. Pat. No. 5,060,063, issued Oct. 21, 1991 to Freeman, and in U.S. Pat. No. Re. 34,615, issued Jan. 31, 1992, also to Freeman, each of which is incorporated herein by reference.
Once the circuit board has been properly aligned with the stencil in the printer, the circuit board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures in the stencil and onto the board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste is typically dispensed onto the stencil from a standard cartridge such as that manufactured by SEMCO Corporation.
In some prior art stencil printers, any excess solder paste remaining under the squeegee after it has fully traversed the stencil, remains on the stencil when the squeegee is returned to its initial position for printing on a second circuit board. In some prior art screen printers, a second squeegee is used that moves across the stencil in the direction opposite to that of the first squeegee. The first squeegee and the second squeegee are used on alternating boards to continually pass the roll of solder paste over the apertures of a stencil to print each successive circuit board. In the prior art stencil printers that utilize two squeegees, there is still the problem that at the end of a manufacturing day, or when the stencil is to be changed, excess solder paste typically remains on the stencil and must be manually removed. Also, in these prior art printers, it is difficult to maintain a desirable viscosity because volatile solvents escape from the solder paste thereby affecting the viscosity of the solder paste.
In the prior art stencil printers discussed above, the squeegee blades are typically at a predetermined angle with the stencil to apply downward pressure on the solder paste to force the solder paste through the apertures in the stencil as the squeegee is moved across the stencil. The angle of the blade is selected based on the speed at which the blade traverses the stencil and based on the desired downward pressure on the solder paste from the blade. It is desirable to maintain a consistent pressure on the solder paste as the squeegee traverses the stencil, however, in typical prior art printers, the pressure varies due to variations in paste viscosity throughout a production run, and due to variations in the angle of the squeegee caused by deformation of the squeegee due to the pressure applied by the squeegee driving device.
It is desirable to provide a method and apparatus for dispensing material onto a stencil of a printer that overcome the problems discussed above.
Embodiments of the present invention provide methods for dispensing material and stencil printers having a dispensing apparatus that overcome the problems of the prior art discussed above.
In a first embodiment of the present invention, a printer for printing a viscous material at predetermined positions forming a pattern on a substrate is provided. The printer includes a frame, a device, mounted to the frame, having a number of perforations arranged to form the pattern, a support apparatus, coupled to the frame that supports the substrate in a printing position beneath the device, and a material dispenser having a substantially cylindrical chamber to contain viscous material to be printed on the device. The chamber has an opening through which the viscous material is dispensed. The material dispenser is coupled to a frame, positioned over the device, and constructed and arranged to dispense the viscous material through the perforations in the device and onto the substrate.
Alternate versions of the first embodiment of the present invention include a number of different features. In one version, the material dispenser is constructed and arranged to be movable along a first axis across the device while the viscous material is being dispensed from the chamber. In another version, the chamber has a cylindrical axis extending along a length of the chamber, and the interior surface of the chamber is coated with a coating material having a low coefficient of friction to allow mixing of the viscous material within the chamber when the material dispenser is moved across the device. In alternate embodiments, different coatings having both high and low coefficients of friction may be used on the interior surface of the chamber to enhance the laminar flow of material in the chamber.
In another version of the first embodiment, the printer further includes a heater to heat the viscous material or a cooler to cool the material, and the material dispenser includes at least one port to receive pressurized paste to increase the paste pressure in the chamber to force viscous material from the chamber.
In yet another version of the first embodiment, the material dispenser further includes a pressure sensor that senses pressure within the chamber, and the printer further includes a controller, coupled to the pressure sensor, that senses the pressure within the chamber and maintains the pressure at a desired value.
In another version of the first embodiment, the material dispenser is adapted to receive a removable cartridge, and in some versions, the removable cartridge is a standard SEMCO cartridge.
In still another version of the first embodiment, the material dispenser includes a pair of inwardly facing blades with side dams that contact the device during printing to prevent excess material from accumulating on the device.
In another version of the first embodiment, the printer further includes a solder gathering squeegee arm that collects excess viscous material remaining on the device when the material dispenser is lifted off of the device.
In a second embodiment of the present invention, a printer for printing a viscous material at predetermined positions forming a pattern on a substrate is provided. The printer includes a frame, a device, mounted to the frame, having a number of perforations arranged to form a pattern, a support apparatus that supports the substrate in a printing position beneath the device, and a material dispenser having a chamber to contain the viscous material to be printed on the substrate. The chamber has an opening through which the viscous material is dispensed. The material dispenser is positioned over the device, constructed and arranged to dispense the viscous material through the perforations in the device and onto the substrate, and adapted to receive a removable cartridge that supplies the viscous material to the chamber. The chamber has an inlet adapted to receive the viscous material from the removable cartridge.
Alternate versions of the second embodiment of the present invention may include one or more of the features of versions of the first embodiment discussed above.
A third embodiment of the present invention provides a material dispenser for dispensing a viscous material onto a stencil of a printer. The material dispenser includes a substantially cylindrical chamber to contain the viscous material. The chamber has an opening through which the viscous material is dispensed. The material dispenser also includes a pair of inwardly facing blades and side dams mounted on the material dispenser in close proximity to the opening. The blades are adapted to contact the stencil during printing to prevent excess material from remaining on the device.
In one version of the material dispenser, an interior surface of the chamber is coated with a coating material having a low coefficient of friction to allow mixing of the viscous material within the chamber when the material dispenser is moved across the device.
A fourth embodiment of the present invention provides a printer for printing a viscous material at predetermined positions forming a pattern on a substrate. The printer includes a frame, a device, mounted to the frame, having a number of perforations arranged to form the pattern, a support apparatus, coupled to the frame, that supports the substrate in a printing position beneath the device and a material dispenser having a chamber to contain the viscous material to be printed on the substrate. The chamber has an opening through which the viscous material is dispensed. The material dispenser is coupled to the frame, positioned over the device, and constructed and arranged to dispense the viscous material through the perforations in the device and onto the substrate. The material dispenser has a retraction device that prevents leakage of the viscous material from the opening after dispensing is complete.
In one version, the retraction device includes at least one plunger that is extendable into the chamber to vary the volume of the chamber, and the retraction device can include an actuator coupled to the plunger to control movement of the plunger. The actuator may have a first air inlet and a second inlet, and the actuator can be constructed and arranged to provide movement of the plunger in a first direction upon the application of pressurized air at the first inlet and to provide movement of the plunger in a second direction upon the application of pressurized air at the second inlet.
In versions of the fourth embodiment, the material dispenser can include at least one port to receive pressurized air to provide air pressure to the chamber to force the viscous material from the chamber during dispensing, and the material dispenser can include a pressure sensor to sense pressure within the chamber, and the printer can further include a controller coupled to the pressure sensor that senses the pressure within the chamber and controls the pressurized air provided through the port to maintain the pressure within the chamber at a desired value. The printer can be adapted to receive a removable cartridge that supplies the viscous material to the chamber, and the chamber can have an inlet to receive the viscous material from the removable cartridge. The pressurized air can be provided to the cartridge to force the viscous material from the cartridge into the chamber.
In a fifth embodiment, a printer is provided for printing a viscous material at predetermined positions forming a pattern on a substrate. The printer includes a frame, a device, mounted to the frame, having a number of perforations arranged to form the pattern, a support apparatus, coupled to the frame, that supports the substrate in a printing position beneath the device, and a material dispenser having a chamber to contain the viscous material to be printed on the substrate. The chamber has an opening through which the viscous material is dispensed, and the material dispenser is coupled to the frame, positioned over the device, and constructed and arranged to dispense the viscous material through the perforations in the device and onto the substrate. The printer also includes means for retaining the viscous material in the chamber after dispensing is complete.
In versions of the fifth embodiment, the means for retaining include means for increasing a volume of the chamber, and the material dispenser is adapted to receive a removable cartridge that supplies the viscous material to the chamber, the chamber having an inlet to receive the viscous material from the removable cartridge. The printer can further include means for applying pressure to the viscous material in the cartridge to force viscous material into the chamber from the cartridge and to force viscous material from the chamber through the opening.
In a sixth embodiment of the present invention, a method is provided for printing a pattern of viscous material on a substrate using a dispenser having a chamber with an inlet to receive viscous material and an outlet from which viscous material is dispensed. The method includes steps of positioning the dispenser over the substrate, applying pressure to the viscous material in the chamber to dispense viscous material from the outlet of the chamber onto the substrate, relieving the pressure applied to the viscous material to discontinue dispensing, and expanding the volume of the chamber to retain the viscous material in the chamber.
The method of the sixth embodiment can further include steps of decreasing the volume of the chamber prior to dispensing material onto the substrate, and coupling a removable cartridge containing the viscous material to the dispenser such that an outlet of the removable cartridge is coupled to the inlet of the chamber. In versions of the sixth embodiment, in the step of applying pressure, air pressure is supplied to the removable cartridge.
In a seventh embodiment of the present invention, a closed loop pressure control system is provided to maintain the pressure of the chamber of the dispensing head to a desired value during printing. The pressure control system includes a pressure transducer mounted in the dispensing head to measure real-time pressure in the chamber at a multiple of predetermined time intervals during the print stroke of the dispensing head. The pressure transducer provides an output signal to a programmable regulator which determines the presence of a pressure differential from a predetermined desired pressure value received from the stencil printer controller. In response to a pressure differential, the programmable regulator increases or decreases the pressurized air delivered to the cartridges during the print stroke. Through control of the pressurized air applied to the cartridges, the programmable regulator controls the pressure of the chamber of the dispensing head to the desired value.
In a version of the seventh embodiment of the present invention, the stencil printer features a bracket mounted to the printer adjacent to the dispensing head which features a universal pressure connection configuration. The universal pressure connection configuration includes a pressure connection mounted on the bracket for connection to similarly configured and arranged connectors of air lines which deliver pressurized air to the cartridges and the actuators of the retraction system. The pressure connection is universal in the sense that it permits the use of different types of dispensing heads which incorporate different pneumatic assemblies and configurations, such as the dispensing head of the present invention and a dispensing head which features one or more squeegee blades, with the stencil printer. The universal pressure connection permits a quick and easy in-process replacement of one type of dispensing head for another, thereby reducing the time required to change dispensing heads. The universal pressure connection configuration results in less production downtime and greater efficiency of the stencil printer.
In an eighth embodiment of the present invention, a temperature control system is provided to control the temperature of the viscous material contained in the chamber of the dispensing head. The temperature control system includes a temperature sensor or thermistor mounted in the dispensing head which measures the temperature of the viscous material. The thermistor provides a signal to a PID controller which determines the presence of a temperature differential from a predetermined desired temperature value received by the printer controller. Depending upon the temperature differential determined, the PID controller indicates to a thermoelectric device disposed at each terminal end of the dispensing head to either heat or cool a conduction rod. The conduction rod is an elongated, insulated rod disposed in the chamber of the dispensing head and coupled at each terminal end to a thermoelectric device. Each thermoelectric device is also coupled to a heat exchanger with a cooling fan which assists in removing heat from the chamber of the dispensing head by convection cooling. The temperature control system maintains the temperature of the viscous material contained in the chamber of the dispensing head to xc2x10.2xc2x0 C. of the desired value.
In a ninth embodiment of the present invention, the dispensing head is equipped with a device which prevents leakage of the viscous material from the chamber after the dispensing head has completed a print stroke or when the dispensing head is lifted from the stencil or substrate. In one version of the ninth embodiment, the device is a blocking assembly disposed in the dispensing head for use in the application of adhesives to substrates. The blocking assembly is mounted to the housing and replaces the retraction system of prior embodiments. The blocking assembly includes an elongated mounting bracket with a plurality of plungers mounted thereon. The blocking assembly conforms to the upper surface of the chamber to prevent leakage of adhesive from the chamber.
In another version of the ninth embodiment of the present invention, the device which prevents leakage of the viscous material from the chamber of the dispensing head is a material holding plate for use with low viscosity materials. Low viscosity materials may not respond as effectively to the negative chamber pressure created by the retraction system and may leak from the dispensing head when the dispensing head is lifted from the stencil or substrate after printing is accomplished. The material holding plate is an elongate, planar member having an upper surface and a lower surface with a plurality of apertures or through-holes. The material holding plate is disposed in the chamber of the dispensing head by sliding the material holding plate into the chamber and matching angled sides of the material holding plate with an interface of a chamber side wall and a squeegee blade mounted to the dispensing head. The material holding plate effectively retains low viscosity materials in the dispensing head and reduces residual viscous material on a stencil or substrate.
In a tenth embodiment of the present invention, the cartridges are mounted to the housing of the dispensing head by stainless steel removable fittings which quickly and easily mount and remove the cartridges from the housing. Each removable fitting includes a smooth insert which couples with a port in the housing having a smooth side wall. Each removable fitting also includes a spring-biased plunger which couples with a stainless steel pad permanently affixed adjacent to the port on the housing. The coupling of the spring-biased plunger and the stainless steel pad creates a secure attachment of the removable fitting to the housing. Each removable fitting eliminates the use of threaded tap fittings and ports for mounting the cartridges which are difficult to clean and require more time to mount the cartridges to the housing.
In addition, each removable fitting enables the cartridges to be mounted to the housing in a vertically and horizontally consistent position in relation to the proximity detector. The proximity detector, as described above, detects the level of paste contained in the cartridge. The consistent position of the cartridge in relation to the proximity detector eliminates erroneous measurements of paste level and permits more reliable paste level detection between cartridges concurrently mounted to the housing and between manufacturing production runs.
In an eleventh embodiment of the present invention, a method is provided to control pressure in the chamber of the material dispenser of the printer. The method includes the steps of applying pressurized air to the chamber from an external source, measuring the pressure of the chamber after at least one predetermined time interval as the material dispenser traverses a stencil or substrate, and either increasing or decreasing the amount of pressurized air applied to the material dispenser in response to detecting a pressure differential between the measured pressure and a predetermined desired pressure to maintain the desired pressure of the chamber.
In a twelfth embodiment of the present invention, a method is provided to control the temperature of the viscous material contained in the chamber of the material dispenser. The method includes the steps of measuring a temperature of the viscous material contained in the chamber, detecting a temperature differential between the measured temperature and a predetermined desired temperature and, in response to the temperature differential, either applying a sufficient amount of heat to increase the temperature the viscous material or removing a sufficient amount of heat to reduce the temperature of the viscous material to maintain the viscous material at the desired temperature.